1. Field of the Invention
This invention relates to the manufacture of flat glass. More particularly the invention relates to the manufacture of thin flat glass by the float process, for example float glass of thickness in the range 1.5 mm to 5 mm and more especially in the range 1.5 mm to 3 mm.
2. Description of the Prior Art
In the float process for flat glass manufacture, molten glass is delivered at a controlled rate on to one end, the hot end, of a molten metal bath contained in an elongated tank structure. Usually the molten metal bath is of molten tin or of a molten tin alloy in which tin predominates. The final ribbon of glass is discharged from the bath by traction means, usually driven traction rollers, disposed beyond the outlet end of the bath, which traction means applies tractive force to advance the ribbon along the bath.
In some ways of operating the float process, regulation of the applied tractive effort is effected along with regulation of the thermal conditions to which the advancing ribbon of glass in subjected so as to attenuate the ribbon to a desired width and thickness. Outwardly and longitudinally directed marginal forces may be applied to the glass while it is being attenuated so as to control gradual and progressive reduction of width and thickness of the ribbon until the ribbon of glass reaches a desired width and thickness. The thermal control is such that when the desired width and thickness of the ribbon is achieved, the velocity of the glass is at a value at which further dimensional change cannot take place under the applied tractive effort.
It has previously been found, particularly when operating under high load conditions, for example at a rate of delivery of molten glass to the bath of 2,000 tonnes per week or more, that a high speed of discharge of the ultimate ribbon of glass from the bath, for example greater than 10 metres per minute, is necessary when attenuating the glass to thicknesses for example below 3mm. It has further been found that the advancing ribbon of glass, when it is accelerating during attenuation to a uniform high speed for discharge from the bath, entrains an appreciable quantity of the molten metal of the bath along the bath surface towards the outlet end of the bath, which surface flow induces an upstream return flow of cooler molten metal from the outlet end of the bath along the bottom of the bath towards the zone of the bath where the ribbon of glass is being attenuated. In this zone the glass is at a viscosity such that it is particularly susceptible to temperature variations across the surface of the molten metal bath, and it has been found that distortion introduced into the underface of the ribbon of glass in this attenuation zone is present in the ultimate ribbon.
Temperature variations across the surface of the bath can result from a temperature gradient through the depth of the bath and it is therefore desirable to minimise such temperature gradients, particularly in the attenuation zone. However, the problem is encountered that although a relatively small temperature gradient can be achieved by a relatively shallow bath depth at low ribbon speeds, a high ribbon speed over a shallow bath depth produces turbulence in the molten metal from which distortion in the ribbon can result. On the other hand, although a greater bath depth will reduce turbulence at high ribbon speeds, it will inherently give a greater temperature gradient through the bath depth which can thermally introduce distortion into the ribbon.
It has previously been proposed in United States Patent Application Serial No. 527,615, filed Nov. 27, 1975, to combat the introduction of such distortion into the ribbon of glass in the attenuation zone by employing a first barrier at a first location in the region of the downstream end of said attenuation zone to constrain molten metal flow at that location to forward flow of molten metal entrained beneath the ribbon and counterflow of molten metal alongside the ribbon from downstream of that location, and employing a second barrier at a second location spaced upstream from said first location and in the region of maximum acceleration of the glass to constrain molten metal flow at that second location to forward flow of molten metal entrained beneath the accelerating glass and counterflow of molten metal alongside the ribbon from downstream of the second location, there being established lateral access into the region of the bath supporting the ribbon between said first and second locations for said counterflow of molten metal at said first location to ensure replenishment of the molten metal of the bath in the attenuation zone between the first and second locations. It has been found that such arrangement can advantageously reduce the temperature difference between the surface molten metal and the molten metal beneath the surface in the attenuation zone, thereby reducing temperature variations in this zone which tend to introduce distortion into the ribbon.
It is a main object of the present invention to provide simplified control of the temperature of the counterflows of molten metal replenishing the molten metal in the attenuation zone.